1. Field of the Invention
This invention relates to a method for forming a neutron image with a neutron beam carrying image information of an object or a sample, e.g. a neutron beam, which has passed through the object, or a neutron beam, which has been emitted by the sample. This invention particularly relates to a method for forming a neutron image, wherein a plurality of stimulable phosphor sheets are superposed one upon another and used.
2. Description of the Prior Art
In the field of non-destructive inspections, techniques have heretofore been used wherein radiation images of objects are obtained by utilizing neutrons, which have been produced by nuclear reactors, or the like. Such techniques are described in, for example, "Shin Hihakai Kensa Benran" (New Non-Destructive Inspection Handbook), published by The Japanese Society For Non-Destructive Inspection, pp. 205-219. The techniques comprise the steps of: (a) exposing a metal converter, which has been combined with a photographic film, to a neutron beam, which has passed through an object, or a neutron beam which has been emitted by a sample, the neutrons of the neutron beam being converted by the metal converter into radiation, such as .alpha.-rays, .beta.-rays, or .gamma.-rays, and (b) exposing the photographic film to the resulting radiation, an image carried by the neutron beam being thereby formed as a photographic image. The metal converter is provided with a layer containing a substance, such as gadolinium (Gd) or lithium (Li), which has a large cross section of nuclear reaction. Neutrons are absorbed by the layer and are thereby converted into the secondary particles, such as .alpha.-particles or .beta.-particles.
The techniques described above are referred to as the neutron radiography. The neutron radiography has the advantages in that it is possible to obtain radiation images representing image information of moisture, organic substances, or the like, which could not be obtained with the radiography utilizing X-rays or .gamma.-rays. In addition, nonmetals coexisting with metals can be detected accurately.
A method for forming a neutron image, which method is improved over the neutron radiography described above, is disclosed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-28899. In the disclosed method, instead of the photographic film and the metal converter being used, a stimulable phosphor sheet is utilized. The stimulable phosphor sheet is provided with a layer of a substance, which is capable of absorbing neutrons and emitting secondary particles, and a layer of a stimulable phosphor, which is capable of storing energy from the secondary particles. When the stimulable phosphor sheet is exposed to a neutron beam carrying image information, the layer of the aforesaid substance absorbs neutrons and emits secondary particles. The stimulable phosphor layer of the stimulable phosphor sheet is exposed to the secondary particles and stored energy from the secondary particles. The stimulable phosphor sheet, on which energy from the secondary particles has been stored, is then scanned in two directions with stimulating rays, which have wavelengths falling within the stimulation wavelength range for the stimulable phosphor sheet and which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the neutron beam. The emitted light is detected photoelectrically, and an image signal representing the neutron image is thereby obtained.
The method for forming a neutron image, wherein a stimulable phosphor sheet is utilized, has the advantages over the neutron radiography where the photographic film and the metal converter are utilized, in that it is possible to form a neutron image having a high sensitivity, a high resolution, and a large area. In addition, that the dose of the neutron beam can be restricted at a low level, and the image signal representing a neutron image can be obtained directly.
A neutron beam has a high penetrating power and readily penetrates through a stimulable phosphor sheet. Therefore, the neutron beam is not readily absorbed by the stimulable phosphor sheet. Such that the neutron beam can be absorbed easily by the stimulable phosphor sheet, it is considered to increase the thickness of the stimulable phosphor sheet. However, for reasons of the transmission of light, which is emitted by the stimulable phosphor sheet when it is exposed to stimulating rays, the thickness of the stimulable phosphor sheet cannot be rendered very thick. At present, the thickness of the stimulable phosphor sheet is limited to at most a value falling within the range of 100 .mu.m to 300 .mu.m.
Therefore, with the conventional method for forming a neutron image wherein a stimulable phosphor sheet is utilized, the efficiency with which a neutron beam is detected cannot be kept high.
In general, neutron beams often contain .gamma.-rays. Ordinarily, stimulable phosphor sheets have a high sensitivity to .gamma.-rays. Therefore, when a neutron image is stored on a stimulable phosphor sheet, energy from the .gamma.-rays contained in the neutron beam is also stored on the stimulable phosphor sheet. In such cases, an image signal detected from the stimulable phosphor sheet also contains components due to the .gamma.-rays in addition to the image information carried by the neutron beam. Accordingly, it becomes impossible to reproduce an image, which is formed with only the neutron beam, from the image signal.